Duck erythrocytes can return to their original isotonic volume in either hypo-, hyper, or isotonic media from an enlarged or shrunken condition. These changes in cell size involve alterations in the cation content of the cells and isoosmotic shifts in cell water. Duck red cells therefore possess a "volume controlling mechanism" which operates by controlling the cation content of cells and is sensitive to some parameter associated with cell size. The "volume controlling mechanism" is separate from the ouabain-inhibitable Na-K exchange pump.